Aqueous latex based paints for both indoor and outdoor architectural paint use possess a number of advantages, such as, low odor, easy clean-up, and fast drying time, over solvent based architectural paints. However, latex paints suffer from the drawback of exhibiting poor wet adhesion and poor freeze-thaw stability.
The term wet adhesion is used in the paint industry to describe the ability of a paint to retain its adhesive bond to a substrate under wet or high humidity conditions. A deficiency of wet adhesion properties limits the utility of latex paints in areas where humid environments might be encountered, for example, exteriors, bathrooms and kitchens. It also makes surfaces painted with latex paints less scrub resistant than those painted with organic solvent based paints.
Latex based paints generally comprise three components: a film forming emulsion polymer or copolymer (the latex), an aqueous phase (which may contain additives as particular applications dictate, such as, pigments, defoamers and antifreeze agents), and one or more surfactants.
The emulsion polymers used in formulating latex paints usually are all-acrylic copolymers comprising alkyl esters of acrylic and methacrylic acid with minor amounts of acrylic and methacrylic acid, or they are vinyl-acrylic polymers comprising vinyl containing monomers or polymers in combination with softer acrylic monomers.
The all-acrylic paints produce hard coatings resistant to scrubbing and provide good water resistance and wet adhesion; but because of the cost of the acrylic monomers, they are expensive. The vinyl-acrylic paints are less expensive and produce films that are hard, but they tend to have a much lower wet adhesion than their acrylic based counterparts.
In recent years improvements in wet adhesion properties have been obtained by the incorporation of amine, amide, and acetoacetate functionalities into the latex polymers. For example, cyclic ureido derivatives have been described in U.S. Pat. Nos. 4,104,220, 4,111,877, 4,219,454, 4,319,032 and 4,599,417 as imparting wet adhesion properties. However, these wet adhesion monomers are very expensive and their inclusion results in a drastic increase in the cost of the vinyl-acrylic polymers and all-acrylic polymers used in the latex based paints.
Latex paints also destabilize on repeated cycles of freezing and thawing. During freeze-thaw cycling the anionic surfactants commonly employed in latex paints tend to desorp from the latex particles, increasing the ionic strength of the aqueous phase and ultimately resulting in paint coagulation. Ordinarily, antifreeze compounds, such as ethylene glycol, are added to the paints to combat the destabilization, but this contributes to the cost of the paint and also results in the use of volatile organic compounds.
Blends of surfactants, such as described in U.S. Pat. No. 3,193,517, have been used in attempts to alleviate this problem. However, the blended system is not entirely satisfactory in other respects and there exists a real need for both wet adhesion promoters and freeze-thaw stabilizers for use in latex paints.